The primary requirement of the laser modulator in a gray scale printer is to provide an adequate palette of the obtainable exposure space to control a predetermined number of exposure steps using the electrophotographic process. The steps are determined by the color space selected and the halftone patterns chosen for imaging.
Basic methods of laser modulation are well known. These methods are pulse width modulation, pulse number modulation and amplitude or intensity modulation. For pulse width modulation, the current supplied to the diode is constant. The modulation is done by changing the "on" time of the laser. For binary monochrome laser printers, a single "on" time or pulse width is used for each pixel. Gray scale printing requires different pulse widths for different gray levels. Pulse number modulation is similar to pulse width modulation in that the current supplied to the laser diode is constant. In gray scale printing, the desired exposure level is reached by the number of times the laser is cycled on and off during the pixel time. In order to cycle the laser, there must be a clock signal whose frequency is many times the frequency of the pixel clock. The high frequency clock signals are gated to the diode using counters for pulse number control. For amplitude or intensity modulation, the "on" time of the laser is constant. The "on" time is the pixel time. The modulation is accomplished by changing the current supplied to the diode. The intensity of the laser diode changes linearly with the current supplied to the diode.
The forward current and power characteristics of a given laser diode is usually provided by the manufacturer in the form of a graph like that shown in FIG. 3 which illustrates the characteristic of a 5 mW laser diode commonly used for laser printing. The laser threshold current is the level of current at which the diode begins to lase. The threshold current is determined graphically in FIG. 3 by drawing an asymptote to the upper linear portion of the characteristics. The intersection of the asymptote and the current axis gives the laser threshold current. As can be seen from the graph in FIG. 3, the current is sensitive to the temperature of the diode. The threshold current of the laser diode increases with temperature as illustrated in FIG. 4.
The increase in threshold current causes a severe drop in light output power. The reduced light output power may not expose the photoconductor to the required amount of energy for the desired output density. For the monochrome, binary printer, the thermal dependency on the laser diode can be minimized by setting the "on" state current to a much higher level than needed at nominal temperature. The current is high enough so that even with the thermally induced power reduction, the light output power is sufficient to expose the photoconductor down to the required level. For a gray scale laser printer which uses intensity or current modulation, overpowering the photoconductor at nominal conditions is not an option. The reason for this is that in intensity modulated systems, the current is the parameter which is changed to obtain the gray scale. It is common to use thermoelectric coolers with intensity modulated laser diodes. A thermoelectric cooler attempts to maintain the laser diode at a constant temperature regardless of ambient conditions. Thermoelectric coolers are custom designed for each application and are, therefore, expensive.
U.S. Pat. No. 4,995,045 is directed to the use of the back facet PIN diode of the laser for feedback in the control of a laser diode source in optical data communications. The patent describes a dual feedback technique used to maintain an average output power of the diode source regardless of the environmental and manufacturing variations of the diode. This, however, does not provide for the direct thermal input into the control of the laser; use of a temperature sensor can provide such an input.
U.S. Pat. No. 4,625,105 describes a technique which diverts a small fraction of the laser light into an external PIN type sensor for feedback purposes in the conversion of an electrical signal into an optical signal. Such an arrangement does not allow for the correction of back facet efficiency drift errors resulting from temperature changes.
The present invention provides thermal compensation for laser diodes used in intensity modulated applications by using feedback from the back facet PIN diode of the laser diode.